GMS v129 - Kanna

And this was one of the ritual's highlights, eh?

But what was the ritual for?

Yes, that was Kanna destroying the Altar. that weakened the ritual and allowed the princess to escape (as seen in Hayato's video)
Nobunaga was using the ritual and the princess's powers to become the 6th Sky Demon King or something. Text in game says that the Demon Sky is another world, like how Grandis is to Maple World, or Japan is to Maple World and that he was opening some kind of portal, which backfired and sent everyone to the wrong world.

There's this tradeable Kanna overall for all classes. Anyone know where you can get this?

That isn't in game from what I can see. The one you start with is untradeable